This application addresses the general problem of tissue and organ regeneration in higher vertebrates, including man. Our studies are guided by the general understanding that regeneration and development share similar mechanisms, and that regenerative ability is enhanced in the embryo and declines with developmental age. Our studies focus on regeneration of developing limb tissues using as models the chick limb bud and the mouse digit tip. These two regeneration models are distinct in that an innate regeneration response in the mouse lends itself to molecular genetic studies to uncover essential signaling pathways, whereas the chick limb bud, which lacks any regenerative ability whatsoever, can be induced to regenerate. Two of the four specific aims focus on the induction of a regeneration response in the chick limb bud (Aims 1 and 2), and the remaining two aims target our continuing effort to dissect signaling pathways important for fetal digit regeneration in the mouse (Aims 3 and 4). In aim 1 we will test the role that cell migration plays in the recruitment of cells to participate in the regeneration response. The induction of a regeneration response will be studied with respect to the role of 1) FGF4 signaling by the AER, 2) signaling by injured tissue to maintain AER function, and 3) a requirement of hyaluronan production. In aim 2 we will carry out functional studies to determine whether signaling involving Msx1 or ErbB1 is required for induced regeneration. Preliminary studies from the Dealy lab (Project 2) on ErbB 1, and recent studies with the mouse Msxl mutant suggest that these signaling pathways will play important roles in the regeneration response. In aim 3 we will extend our previous studies showing that BMP signaling is required for fetal digit tip regeneration. Targeting the type I Bmp receptors we will carry out loss of function and gain of function studies in cell transplantation assays in wildtype digits. These studies will determine whether BMP signaling essential for cells to participate in a regeneration response. In aim 4 we will investigate the role that Dlx5, a homeodomain protein expressed in the digit tip, plays in the regeneration response. Regeneration studies using the Dlx5 mutant and the Dlx5/6 double mutant digit will determine whether these Dlx genes are required for a regeneration response.